An important characteristic in the evaluation of the quality and performance of textured yarns is crimp recovery potential, which is defined as the amount by which a textured yarn decreases in length in a fully relaxed condition when allowed to retract from the fully extended state. Textured yarn is formed of one or more continuous filaments in a first-set state which are false-twisted, crimped, curled, or otherwise compacted while heated to set a kink into the filaments which will remain when the filaments are cooled. The textured yarn now has a second-set state and has retracted to a shorter overall length, Usually the yarn is wound onto a package, which may in time impose yet another, third set state. This third-set state is eliminated by developing the yarn either while it is still in yarn form or after it has been formed into a fabric. Developing is a technique by which the yarn or fabric made of the yarn is relaxed by heating to a temperature below that used to produce the second-set state while the yarn or fabric is fully relaxed, or at least partially relaxed to some preset standard. During this development process, a second, separate action known as fiber shrinkage may occur due to the heating of the yarn. Thus, while removing the effects of the third-set state or packaging-set state, a real axial shrinkage may be induced. The length of the yarn in the developed relaxed state is thus the original length of the yarn in its fully extended state minus the sum of the crimp recovery plus the fiber shrinkage. The term "relaxed" is used to indicate both a fully relaxed yarn and also a yarn which is lightly tensioned as understood in the art.
A number of attempts have been made to measure recovery potentials in textured yarn. In one approach, the recovery potential of a textured yarn is determined by first developing and then hanging a number of different weights on a skein of yarn to measure skein lengths to calculate crimp recovery and fiber shrinkage. The process is not continuous and requires a substantial amount of time. In another system a fully extended yarn is heated in a zone between two feed roll systems, with the downstream feeder fixed to operate at a predetermined slower feed rate than the upstream feeder so that partial retraction takes place in the test zone. Means are used to measure the tension in the test zone as an indicator of recovery potential, but since the speed of the downstream feeder is fixed at a preset level, it produces only an indication of the yarn's performance under preselected conditions and is not an absolute measure of the yarn's recovery potentials. This approach also does not distinguish the crimp recovery from the fiber shrinkage. The instant inventor proposed a system in which a fully extended yarn was fed into a controlled tension zone and over a heater between upstream and downstream feed rolls. A spring-loaded arm varied the speed of the upstream feeder to maintain the constant tension on the yarn in the test zone, and the rotational speeds of the feed rolls were compared to each other to determine the extent of total retraction that had occurred in the test zone. The device failed to separate crimp recovery and fiber shrinkage and the device was not capable of controlling the tension at low tension levels, e.g. one to two milligrams per denier. In another, two-zone device, development took place in the first zone between two sets of feeders moving at speeds corresponding to the recovery level of the yarn that moved between them. A second zone was established between the second feeder rolls and a third set of feeder rolls. The third set of feeder rolls operated at the same rate as the first set of feeder rolls. A tensiometer engaged the yarn to measure the force required to stretch the yarn in the second zone back to its undeveloped length. An increase in tension in the second zone over and above the initial known input tension would be an indication that there had been some fiber shrinkage. After separate stress-strain studies of the test yarns, approximate shrinkage length might be obtained for specific tension values. The tension determination was extremely sensitive to variations in the input tension to the first feeder. Absolute values of crimp recovery and fiber shrinkage were not obtained, and the system was not sensitive nor reliable enough. For example, the damping required on the tension arm introduced extraneous forces to the yarn tension upon changes in recovery of the yarn. This interfered with uniformity of tension during the yarn development, which uniformity is necessary for meaningful results.
A shortcoming in a number of these devices is the tensioning devices which have been used to attempt to maintain the tension on the yarn which is so necessary for accurate, meaningful results. In the past they have tended to be not sensitive or reliable enough for measuring recovery and shrinkage at low-tension levels. Often damping or other approaches introduced extraneous forces to the yarn tensioning system and interfered with the useful results.